Memory circuit and memory device

ABSTRACT

To reduce power consumption, a memory circuit includes a latch unit in which first data and second data are rewritten and read in accordance with a control signal, a first switch unit that controls rewrite and read of the first data stored in the latch unit by being turned on or off in response to the control signal, and a second switch unit that controls rewrite and read of the second data stored in the latch unit by being turned on or off in response to the control signal. The latch unit includes a first inverter and a second inverter. At least one of the first inverter and the second inverter includes a first field-effect transistor, and a second field-effect transistor that has the same conductivity type as the first field-effect transistor and has a gate potential controlled in accordance with the control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/683,257, filed Nov. 21, 2012, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2011-256890 on Nov. 25, 2011, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a memory circuit.Further, one embodiment of the present invention relates to a memorydevice including a memory circuit.

2. Description of the Related Art

In recent years, devices including circuits composed of only transistorshaving the same conductivity type (also referred to as circuits composedof only n-channel transistors or p-channel transistors) have beendeveloped.

An example of the circuit composed of only transistors having the sameconductivity type is an inverter, which is a logic gate.

For example, when the mobility is very different between a p-channeltransistor and an n-channel transistor that both include a channelformation region containing the same semiconductor material, it isdifficult to constitute an inverter using these p-channel and re-channeltransistors. In contrast, an inverter can be easily constituted oftransistors having the same conductivity type.

An example of a device including the inverter is a memory circuit (e.g.,Patent Document 1).

For example, a memory circuit disclosed in Patent Document 1 is a memorycell of static random access memory (SRAM). The memory circuit disclosedin Patent Document 1 includes two switching transistors and twoinverters each of which is composed of only transistors having the sameconductivity type.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    H7-282584

SUMMARY OF THE INVENTION

A conventional memory circuit has a problem of large power consumption.

For example, in the memory circuit disclosed in Patent Document 1,current always flows between a source and a drain of the depletiontransistor included in either inverter and leakage current flows evenwhile data is held. As a result, power consumption is increased as thecapacity of the SRAM is increased, for example.

Moreover, in the memory circuit disclosed in Patent Document 1, storeddata is lost when supply of the power supply voltage stops. Accordingly,the power supply voltage needs to continue to be supplied at all timeswhile data is held, which increases power consumption.

An object of one embodiment of the present invention is to reduce powerconsumption.

In one embodiment of the present invention, a memory circuit isconstituted using an inverter composed of two field-effect transistorshaving the same conductivity type. One of the two field-effecttransistors is a field-effect transistor in which the amount of currentbetween a source and a drain is changed in response to a control signal.The above structure can reduce the leakage current in a hold period, forexample.

According to one embodiment of the present invention, a memory circuitincludes a latch circuit, a first switch unit, and a second switch unit.The latch unit has a function of storing first data and second data. Thefirst data and the second data are rewritten into and read from thelatch unit in accordance with a control signal. The first switch unitcontrols rewrite and read of the first data stored in the latch unit, bybeing turned on or off in response to the control signal. The secondswitch unit controls rewrite and read of the second data stored in thelatch unit, by being turned on or off in response to the control signal.The latch unit includes a first inverter and a second inverter. Thepotential of an input terminal of the first inverter is the first data.The first inverter maintains a value of the second data in response to apotential of an output terminal of the first inverter. The potential ofan input terminal of the second inverter is the second data. The secondinverter maintains a value of the first data in response to a potentialof an output terminal of the second inverter. At least one of the firstinverter and the second inverter includes a first field-effecttransistor and a second field-effect transistor. A gate of the firstfield-effect transistor serves as the input terminal of the inverter.The first field-effect transistor controls whether the potential of theoutput terminal of the inverter is set at a first potential or not inaccordance with the potential of the input terminal of the inverter. Thesecond field-effect transistor has the same conductivity type as thefirst field-effect transistor. The gate potential of the secondfield-effect transistor is controlled in accordance with the controlsignal. The second field-effect transistor controls whether thepotential of the output terminal of the inverter is set at a secondpotential or not in accordance with the control signal.

In one embodiment of the present invention, the memory circuit may beconstituted as a nonvolatile memory circuit by using transistors withlow off-state current, for example, as all of field-effect transistorsincluded in the latch unit, the first switch unit, and the second switchunit. Thus, the memory circuit can hold data even if supply of the powersupply voltage to the memory circuit stops, for example.

According to one embodiment of the present invention, unwanted currentflow in the memory circuit can be suppressed, so that power consumptioncan be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A, 1B, 1C1, and 1C2 illustrate an example of a memory circuit;

FIG. 2 is a timing chart for explaining an example of the memorycircuit;

FIGS. 3A and 3B each illustrate an example of a memory circuit;

FIGS. 4A and 4B each illustrate an example of a memory circuit;

FIGS. 5A and 5B each illustrate an example of a memory circuit;

FIGS. 6A and 6B each illustrate an example of a memory circuit;

FIGS. 7A and 7B each illustrate an example of a memory circuit;

FIG. 8 is a timing chart for explaining an example of the memorycircuit;

FIGS. 9A to 9C are schematic cross-sectional views illustrating anexample of a structure of the memory circuit;

FIG. 10 is a block diagram illustrating an example of a memory device;

FIG. 11 is a block diagram illustrating an example of an arithmeticprocessing unit;

FIGS. 12A to 12D each illustrate an example of an electronic device; and

FIG. 13 illustrates an example of an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described. Notethat it will be readily appreciated by those skilled in the art thatdetails of the embodiments can be modified without departing from thespirit and scope of the present invention. Thus, the present inventionshould not be limited to the description of the following embodiments,for example.

Note that the contents in different embodiments can be combined with oneanother as appropriate. In addition, the contents of the embodiments canbe replaced with each other as appropriate.

Ordinals such as “first” and “second” are used in order to avoidconfusion among components, and the number of components is not limitedby the number of ordinals.

Embodiment 1

In this embodiment, an example of a memory circuit including an invertercomposed of only transistors having the same conductivity type will bedescribed with reference to FIGS. 1A, 1B, 1C1, and 1C2, FIG. 2, andFIGS. 3A and 3B.

The memory circuit in this embodiment includes a latch unit Lat, aswitch unit SwA, and a switch unit SwB, for example, as illustrated inFIG. 1A.

The latch unit Lat is supplied with the power supply voltage through afirst power supply line PSL1 and a second power supply line PSL2. Thelatch unit Lat is also supplied with a control signal WR through arewrite/read control signal line WRL. The latch unit Lat has a functionof storing first data D1 and second data D2. The first data D1 and thesecond data D2 are rewritten and read in accordance with the controlsignal WR.

The switch unit SwA and the switch unit SwB are supplied with thecontrol signal WR through the rewrite/read control signal line WRL.

The switch unit SwA has a function of controlling rewrite and read ofthe first data D1 in accordance with the control signal WR.

The switch unit SwB has a function of controlling rewrite and read ofthe second data D2 in accordance with the control signal WR.

Each of the switch units SwA and SwB has, for example, a field-effecttransistor whose on/off state is controlled in accordance with thecontrol signal WR.

As illustrated in FIG. 1B, the latch unit Lat includes a first inverterInv1 and a second inverter Inv2.

The potential of an input terminal of the first inverter Inv1 is thefirst data D1. The value of the second data D2 is maintained in responseto the potential of an output terminal of the first inverter Inv1.

The potential of an input terminal of the second inverter Inv2 is thesecond data D2. The value of the first data D1 is maintained in responseto the potential of an output terminal of the second inverter Inv2.

An inverter (also referred to as inverter Inv) that is at least one ofthe first inverter Inv1 and the second inverter Inv2 is composed of onlytransistors having the same conductivity type, and includes a firstfield-effect transistor Tr1 and a second field-effect transistor Tr2 asillustrated in FIGS. 1C1 and 1C2.

The potential of one of a source and a drain of the first field-effecttransistor Tr1 is set based on the potential of the first power supplyline PSL1. A gate of the first field-effect transistor Tr1 serves as aninput terminal (also referred to as terminal DIn) of the inverter Inv.The first field-effect transistor Tr1 has a function of controllingwhether the potential of an output terminal (also referred to asterminal DOut) of the inverter Inv is set at a first potential V1 inaccordance with the potential of the input terminal (the terminal DIn)of the inverter Inv.

The second field-effect transistor Tr2 has the same conductivity type asthe first field-effect transistor Tr1. The potential of one of a sourceand a drain of the second field-effect transistor Tr2 is set based onthe potential of the second power supply line PSL2. The potential of agate of the second field-effect transistor Tr2 is controlled inaccordance with the control signal WR. The second field-effecttransistor Tr2 has a function of controlling whether the potential ofthe output terminal (the terminal DOut) of the inverter Inv is set at asecond potential V2.

Note that the levels of the first potential V1 and the second potentialV2 are determined by the ratio of the amount of current flowing betweenthe source and the drain of the first field-effect transistor Tr1 to theamount of current flowing between the source and the drain of the secondfield-effect transistor Tr2.

For example, the other of the source and the drain of the firstfield-effect transistor Tr1 illustrated in FIGS. 1C1 and 1C2 has thepotential of the output terminal (the terminal DOut) of the inverterInv. The gate of the first field-effect transistor Tr1 illustrated inFIGS. 1C1 and 1C2 has the potential of the input terminal (the terminalDIn) of the inverter Inv.

The other of the source and the drain of the second field-effecttransistor Tr2 illustrated in FIGS. 1C1 and 1C2 is electricallyconnected to the other of the source and the drain of the firstfield-effect transistor Tr1.

The second field-effect transistor Tr2 illustrated in FIG. 1C1 has apair of gates that overlap each other with a channel formation regionplaced therebetween. One of the pair of gates and the other of thesource and the drain of the second field-effect transistor Tr2illustrated in FIG. 1C1 are electrically connected to each other. Thepotential of the other of the pair of gates (also referred to asterminal BG) of the second field-effect transistor Tr2 illustrated inFIG. 1C1 is changed in response to the potential of the control signalWR.

The threshold voltage of the second field-effect transistor Tr2 ischanged in response to the potential of the other of the pair of gatesof the second field-effect transistor Tr2. For example, when thepotential of the other of the pair of gates of the second field-effecttransistor Tr2 increases, the threshold voltage of the secondfield-effect transistor Tr2 decreases. Thus, the potential of the otherof the pair of gates of the second field-effect transistor Tr2 ischanged in response to the potential of the control signal WR, wherebythe threshold voltage of the second field-effect transistor Tr2 can beset in accordance with the on/off state of the switch unit SwA and theswitch unit SwB to which the same control signals WR are input, and forexample, the second field-effect transistor Tr2 can be set in a normallyoff state or a normally on state.

Note that the second field-effect transistor Tr2 does not necessarilyhave a pair of gates. For example, the second field-effect transistorTr2 illustrated in FIG. 1C2 is a normal field-effect transistor, and thepotential of a gate (also referred to as terminal FG) of the secondfield-effect transistor Tr2 is changed in response to the potential ofthe control signal WR.

The above is the description of the configuration example of the memorycircuit in this embodiment.

Next, an example of a method for driving the memory circuit in thisembodiment will be described with reference to a timing chart in FIG. 2.Here, the first inverter Inv1 and the second inverter Inv2 each have theconfiguration illustrated in FIG. 1C1, and all the field-effecttransistors included in the memory circuit of this embodiment aren-channel transistors. The potential of a high-level signal is apotential VH, the potential of a low-level signal is a potential VL, andan undefined (don't care) value is represented by X. The potential ofthe first power supply line PSL1 is the potential VH, and the potentialof the second power supply line PSL2 is the potential VL. A double wavyline in FIG. 2 represents omission.

In the example of a method for driving the memory circuit in thisembodiment, first, the potentials of a first data signal line DL1 and asecond data signal line DL2 are set in a rewrite period (also referredto as period T_WT). At this time, the potential of the first data signalline DL1 is set at DataA, and the potential of the second data signalline DL2 is set at DataB. One of DataA and DataB is a high-levelpotential and the other thereof is a low-level potential. The potentialof the control signal WR is set at a potential VSH, whereby thepotential of the rewrite/read control signal line WRL is set at thepotential VSH. The potential VSH is higher than a ground potential andcan be set as appropriate in accordance with the specifications of thememory circuit. In the case where one of DataA and DataB is data “1” andthe other thereof is data “0”, 1-bit data can be held in the memorycircuit.

In the rewrite period, the switch unit SwA and the switch unit SwB areturned on, and DataA is written as the first data D1 and DataB iswritten as the second data D2 of the latch unit Lat.

In each of the first inverter Inv1 and the second inverter Inv2, thepotential of the other of the pair of gates (the terminal BG) of thesecond field-effect transistor Tr2 is set in response to the potentialof the control signal WR.

The case where the potential of DataA is a high-level potential (thepotential VH) and the potential of DataB is a low-level potential (thepotential VL) is shown below as an example.

In this case, in the first inverter Inv1, the first field-effecttransistor Tr1 is turned on, and the amount of current flowing betweenthe source and the drain of the second field-effect transistor Tr2becomes smaller than the amount of current flowing between the sourceand the drain of the first field-effect transistor Tr1 in response tothe control signal WR.

In the second inverter Inv2, the first field-effect transistor Tr1 isturned off, and the amount of current flowing between the source and thedrain of the second field-effect transistor Tr2 becomes larger than theamount of current flowing between the source and the drain of the firstfield-effect transistor Tr1 in response to the control signal WR. Thus,the potential for the first data D1 can be set by both the switch unitSwA and the field-effect transistor Tr2 included in the second inverterInv2. Accordingly, the speed for writing the first data D1 can beincreased.

Then, in a hold period (also referred to as period T_HLD), the potentialof the control signal WR is set at a potential VSL, whereby thepotential of the rewrite/read control signal line WRL is set at thepotential VSL. The potential VSL is lower than the potential VSH and islower than or equal to a ground potential. The potential VSL can be setas appropriate in accordance with the specifications of the memorycircuit.

At this time, the switch unit SwA and the switch unit SwB are turnedoff, and the first data D1 (DataA) and the second data D2 (DataB)written into the latch unit Lat are held by the first inverter Inv1 andthe second inverter Inv2.

In each of the first inverter Inv1 and the second inverter Inv2, thesecond field-effect transistor Tr2 is turned off in response to thecontrol signal WR. Consequently, leakage current does not flow in thefirst inverter Inv1 and the second inverter Inv2.

Then, in a read period (also referred to as period T_RD), the potentialof the control signal WR is set at the potential VSH, whereby thepotential of the rewrite/read control signal line WRL is set at thepotential VSH.

At this time, the switch unit SwA and the switch unit SwB are turned on,and the potential of the first data signal line DL1 is set in responseto DataA and the potential of the second data signal line DL2 is set inresponse to DataB. Thus, the first data D1 (DataA) and the second dataD2 (DataB) stored in the latch unit Lat are read.

The case where the potential of DataA is a high-level potential (thepotential VH) and the potential of DataB is a low-level potential (thepotential VL) is shown below as an example.

In this case, in the first inverter Inv1, the first field-effecttransistor Tr1 is turned on, and the amount of current flowing betweenthe source and the drain of the second field-effect transistor Tr2becomes smaller than the amount of current flowing between the sourceand the drain of the first field-effect transistor Tr1 in response tothe control signal WR.

In the second inverter Inv2, the first field-effect transistor Tr1 isturned off, and the amount of current flowing between the source and thedrain of the second field-effect transistor Tr2 becomes larger than theamount of current flowing between the source and the drain of the firstfield-effect transistor Tr1 in response to the control signal WR. Thus,in the read period, current flows between the source and the drain ofthe second field-effect transistor Tr2 in the second inverter Inv2,which enables deviation of the value of the first data D1 to becorrected in the read period even if the value of the first data D1 ischanged in the hold period due to leakage current of anotherfield-effect transistor, for example.

The case where the potential of DataA is a low-level potential (thepotential VL) and the potential of DataB is a high-level potential (thepotential VH) is explained below.

In this case, in the first inverter Inv1, the first field-effecttransistor Tr1 is turned off, and the amount of current flowing betweenthe source and the drain of the second field-effect transistor Tr2becomes larger than the amount of current flowing between the source andthe drain of the first field-effect transistor Tr1 in response to thecontrol signal WR. Thus, in the read period, current flows between thesource and the drain of the second field-effect transistor Tr2 in thefirst inverter Inv1, which enables deviation of the value of the seconddata D2 to be corrected in the read period even if the value of thesecond data D2 is changed in the hold period due to leakage current ofanother field-effect transistor, for example.

Further, in the second inverter Inv2, the first field-effect transistorTr1 is turned on, and the amount of current flowing between the sourceand the drain of the second field-effect transistor Tr2 becomes smallerthan the amount of current flowing between the source and the drain ofthe first field-effect transistor Tr1 in response to the control signalWR.

The above is the description of the example of the method for drivingthe memory circuit in this embodiment.

The memory circuit in this embodiment may be a nonvolatile memorycircuit that can hold the first data D1 and the second data D2 even whensupply of power stops. For example, a nonvolatile memory circuit can beformed by using field-effect transistors with low off-state current asall the field-effect transistors included in the latch unit Lat, theswitch unit SwA, and the switch unit SwB. An example of a field-effecttransistor with low off-state current is a field-effect transistor usinga material with a bandgap wider than that of silicon. In this case, theoff-state current of the field-effect transistor per channel width of 1μm is preferably 1 zA or less.

When field-effect transistors with low off-state current are used as allthe field-effect transistors included in the latch unit Lat, the switchunit SwA, and the switch unit SwB, a switch unit SwC may be provided inthe memory circuit as illustrated in FIGS. 3A and 3B.

The switch unit SwC is supplied with the control signal WR through therewrite/read control signal line WRL. The switch unit SwC has a functionof controlling whether to supply the power supply voltage to the latchunit Lat by being turned on or off in response to the control signal WR.

For example, in the case where a potential applied to the second powersupply line PSL2 is higher than a potential applied to the first powersupply line PSL1, the switch unit SwC is provided between the secondpower supply line PSL2 and the latch unit Lat as illustrated in FIG. 3A.Thus, whether the potential of the second power supply line PSL2 issupplied to the latch unit Lat can be controlled in accordance with thecontrol signal WR.

For example, in the case where a potential applied to the first powersupply line PSL1 is higher than a potential applied to the second powersupply line PSL2, the switch unit SwC is provided between the firstpower supply line PSL1 and the latch unit Lat as illustrated in FIG. 3B.Thus, whether the potential of the first power supply line PSL1 issupplied to the latch unit Lat can be controlled in accordance with thecontrol signal WR.

The configuration is not limited to those described above, and theswitch unit SwC may be provided both between the first power supply linePSL1 and the latch unit Lat and between the second power supply linePSL2 and the latch unit Lat.

The switch unit SwC has a field-effect transistor, for example.

Providing the switch unit SwC makes it possible to hold a potential tobe the power supply voltage in the memory circuit even if supply of thepower supply voltage to the memory circuit stops.

The above is the description of the configuration examples of the memorycircuit in this embodiment.

As has been described with FIGS. 1A, 1B, 1C1, and 1C2, FIG. 2, and FIGS.3A and 3B, in the memory circuit according to one example of thisembodiment, the inverter composed of only transistors having the sameconductivity type in the latch unit is formed using a field-effecttransistor, a gate potential of which is controlled in accordance with acontrol signal. Thus, the field-effect transistor can be in a normallyoff state during a non-operating period such as the hold period.Consequently, leakage current of the inverter in the non-operatingperiod can be reduced, and as a result, power consumption of the memorycircuit can be reduced. Further, a control signal used for controllingthe first switch unit and the second switch unit also controls theinverter, whereby the number of signals used in the memory circuit canbe decreased.

In addition, the memory circuit according to one example of thisembodiment is a nonvolatile memory circuit, in which case data writteninto the latch unit can be held even if supply of the power supplyvoltage to the memory circuit stops. Accordingly, power consumption canbe reduced, for example, by stopping supply of the power supply voltageto the memory circuit while data is not rewritten or read.

Embodiment 2

In this embodiment, as a specific example of the memory circuitdescribed in Embodiment 1, a memory circuit constituted only ofn-channel field-effect transistors will be described with reference toFIGS. 4A and 4B, FIGS. 5A and 5B, FIGS. 6A and 6B, FIGS. 7A and 7B, andFIG. 8. Note that the description of the memory circuit in Embodiment 1can apply to the same portions as in Embodiment 1 as appropriate.

FIG. 4A illustrates an example of a memory circuit in this embodiment.The memory circuit illustrated in FIG. 4A includes the latch unit Lat,the switch unit SwA, and the switch unit SwB.

The switch unit SwA has a field-effect transistor 211.

One of a source and a drain of the field-effect transistor 211 iselectrically connected to the first data signal line DL1. A gate of thefield-effect transistor 211 is electrically connected to therewrite/read control signal line WRL.

The switch unit SwB has a field-effect transistor 212.

One of a source and a drain of the field-effect transistor 212 iselectrically connected to the second data signal line DL2. A gate of thefield-effect transistor 212 is electrically connected to therewrite/read control signal line WRL.

The latch unit Lat includes the first inverter Inv1 and the secondinverter Inv2.

The first inverter Inv1 includes a field-effect transistor 213 and afield-effect transistor 214.

One of a source and a drain of the field-effect transistor 213 iselectrically connected to the first power supply line PSL1. The firstpower supply line PSL1 is supplied with the potential VL. A gate of thefield-effect transistor 213 is electrically connected to the other ofthe source and the drain of the field-effect transistor 211.

One of a source and a drain of the field-effect transistor 214 iselectrically connected to the second power supply line PSL2. The secondpower supply line PSL2 is supplied with the potential VH. The other ofthe source and the drain of the field-effect transistor 214 iselectrically connected to the other of the source and the drain of thefield-effect transistor 213. The field-effect transistor 214 has a pairof gates that overlap each other with a channel formation region placedtherebetween. One of the pair of gates and the other of the source andthe drain of the field-effect transistor 214 are electrically connectedto each other. The other of the pair of gates of the field-effecttransistor 214 is electrically connected to the rewrite/read controlsignal line WRL.

The second inverter Inv2 includes a field-effect transistor 215 and afield-effect transistor 216.

One of a source and a drain of the field-effect transistor 215 iselectrically connected to the first power supply line PSL1. The other ofthe source and the drain of the field-effect transistor 215 iselectrically connected to the gate of the field-effect transistor 213. Agate of the field-effect transistor 215 is electrically connected to theother of the source and the drain of the field-effect transistor 212 andthe other of the source and the drain of the field-effect transistor213. Note that the potential of a portion where the other of the sourceand the drain of the field-effect transistor 215 and the gate of thefield-effect transistor 213 are connected to each other is called thefirst data D1. Note also that the potential of a portion where the otherof the source and the drain of the field-effect transistor 213 and thegate of the field-effect transistor 215 are connected to each other iscalled the second data D2.

One of a source and a drain of the field-effect transistor 216 iselectrically connected to the second power supply line PSL2. The otherof the source and the drain of the field-effect transistor 216 iselectrically connected to the other of the source and the drain of thefield-effect transistor 215. The field-effect transistor 216 has a pairof gates that overlap each other with a channel formation region placedtherebetween. One of the pair of gates and the other of the source andthe drain of the field-effect transistor 216 are electrically connectedto each other. The other of the pair of gates of the field-effecttransistor 216 is electrically connected to the rewrite/read controlsignal line WRL.

Note that the timing at which the control signal WR is input to the gateof each of the field-effect transistors 211 and 212 may be later thanthe timing at which the control signal WR is input to the other of thepair of gates of each of the field-effect transistors 214 and 216.Accordingly, even if the value of the first data D1 or the second dataD2 in the latch unit Lat is changed during the hold period, for example,current flows between the source and the drain of the field-effecttransistors 214 and 216 in the read period and the value of the firstdata D1 or the second data D2 is corrected, and then the field-effecttransistors 211 and 212 are turned on, whereby the first data D1 and thesecond data D2 can be read. The timing of inputting the control signalWR can be changed, for example, by providing a delay circuit between thegate of the field-effect transistor 211 and the rewrite/read controlsignal line WRL and between the gate of the field-effect transistor 212and the rewrite/read control signal line WRL.

Alternatively, as illustrated in FIG. 4B, a field-effect transistor 314that is a normal field-effect transistor may be used instead of thefield-effect transistor 214 illustrated in FIG. 4A, and a field-effecttransistor 316 that is a normal field-effect transistor may be usedinstead of the field-effect transistor 216.

In this case, one of a source and a drain of the field-effect transistor314 is electrically connected to the second power supply line PSL2. Theother of the source and the drain of the field-effect transistor 314 iselectrically connected to the other of the source and the drain of thefield-effect transistor 213. A gate of the field-effect transistor 314is electrically connected to the rewrite/read control signal line WRL.

In addition, one of a source and a drain of the field-effect transistor316 is electrically connected to the second power supply line PSL2. Theother of the source and the drain of the field-effect transistor 316 iselectrically connected to the other of the source and the drain of thefield-effect transistor 215. A gate of the field-effect transistor 316is electrically connected to the rewrite/read control signal line WRL.

A nonvolatile memory circuit can be constituted by using transistorswith low off-state current as the field-effect transistors 211 to 216and the field-effect transistors 314 and 316. As a transistor with lowoff-state current, a transistor using a material with a bandgap widerthan that of silicon can be used, for example.

When transistors with low off-state current are used as the field-effecttransistors 211 to 216 and the field-effect transistors 314 and 316, thememory circuit in this embodiment can have configurations illustrated inFIGS. 5A and 5B. The memory circuit illustrated in FIG. 5A includes theswitch unit SwC in addition to the components illustrated in FIG. 4A,and the memory circuit illustrated in FIG. 5B includes the switch unitSwC in addition to the components illustrated in FIG. 4B.

The switch unit SwC has a field-effect transistor 221.

In the switch unit SwC illustrated in FIG. 5A, one of a source and adrain of the field-effect transistor 221 is electrically connected tothe second power supply line PSL2. The other of the source and the drainof the field-effect transistor 221 is electrically connected to the oneof the source and the drain of each of the field-effect transistors 214and 216.

In the switch unit SwC illustrated in FIG. 5B, one of the source and thedrain of the field-effect transistor 221 is electrically connected tothe second power supply line PSL2. The other of the source and the drainof the field-effect transistor 221 is electrically connected to the oneof the source and the drain of each of the field-effect transistors 314and 316.

Alternatively, the memory circuit in this embodiment can haveconfigurations illustrated in FIGS. 6A and 6B. The memory circuitillustrated in FIG. 6A includes a switch unit SwD and a switch unit SwEin addition to the components illustrated in FIG. 4A, and the memorycircuit illustrated in FIG. 6B includes the switch unit SwD and theswitch unit SwE in addition to the components illustrated in FIG. 4B.

The switch unit SwD has a field-effect transistor 231. One of a sourceand a drain of the field-effect transistor 231 is electrically connectedto the gate of the field-effect transistor 213. The other of the sourceand the drain of the field-effect transistor 231 is electricallyconnected to the other of the pair of gates of the field-effecttransistor 216. A gate of the field-effect transistor 231 iselectrically connected to the rewrite/read control signal line WRL.

The switch unit SwE has a field-effect transistor 232. One of a sourceand a drain of the field-effect transistor 232 is electrically connectedto the gate of the field-effect transistor 215. The other of the sourceand the drain of the field-effect transistor 232 is electricallyconnected to the other of the pair of gates of the field-effecttransistor 214. A gate of the field-effect transistor 232 iselectrically connected to the rewrite/read control signal line WRL.

As illustrated in FIG. 7A, the switch units SwD and SwE can beadditionally provided in the configuration in FIG. 5A, or as illustratedin FIG. 7B, the switch units SwD and SwE can be additionally provided inthe configuration in FIG. 5B. The description of the switch units SwDand SwE illustrated in FIGS. 6A and 6B can apply to the switch units SwDand SwE here.

In the configuration where the switch units SwD and SwE are provided,when the field-effect transistors 231 and 232 are on in the rewriteperiod and the read period, the potential of the first data D1 is ahigh-level potential, and the potential of the second data D2 is alow-level potential, the field-effect transistor 214 can be turned offby setting the potential of the other of the pair of gates of thefield-effect transistor 214 at a low-level potential and thefield-effect transistor 216 can be turned on by setting the potential ofthe other of the pair of gates of the field-effect transistor 216 at ahigh-level potential.

Similarly, when the potential of the first data D1 is a low-levelpotential and the potential of the second data D2 is a high-levelpotential in the rewrite period and the read period, the field-effecttransistor 214 can be turned on and the field-effect transistor 216 canbe turned off.

When the potential of the first data D1 is a high-level potential andthe potential of the second data D2 is a low-level potential in therewrite period and the read period, the field-effect transistor 314 canbe turned off and the field-effect transistor 316 can be turned on.

Further, when the potential of the first data D1 is a low-levelpotential and the potential of the second data D2 is a high-levelpotential in the rewrite period and the read period, the field-effecttransistor 314 can be turned on and the field-effect transistor 316 canbe turned off.

By thus providing the switch units SwD and SwE, the field-effecttransistors that do not need to be on in the rewrite period and the readperiod can be turned off in these periods; accordingly, leakage currentcan be suppressed. Consequently, data stored in the rewrite period ordata read in the read period can be prevented from varying due toleakage current generated in the first inverter Inv1 and the secondinverter Inv2. Moreover, power consumption can be reduced.

The above is the description of the configuration examples of the memorycircuit in this embodiment.

Next, as an example of a method for driving the memory circuit in thisembodiment, an example of a method for driving the memory circuitillustrated in FIG. 4A will be described with reference to a timingchart in FIG. 8. Note that the memory circuit is a nonvolatile memorycircuit, the potential of a high-level signal is the potential VH, thepotential of a low-level signal is the potential VL, and an undefined(don't care) value is represented by X. A double wavy line in FIG. 8represents omission.

In the example of a method for driving the memory circuit in thisembodiment, first, the potentials of the first data signal line DL1 andthe second data signal line DL2 are set in the rewrite period (theperiod T_WT). At this time, the potential of the first data signal lineDL1 is set at DataA, and the potential of the second data signal lineDL2 is set at DataB. One of DataA and DataB is a high-level potentialand the other thereof is a low-level potential. The potential of thecontrol signal WR is set at the potential VSH, whereby the potential ofthe rewrite/read control signal line WRL is set at the potential VSH.

In the rewrite period, the field-effect transistors 211 and 212 areturned on, and DataA is written as the first data D1 and DataB iswritten as the second data D2 of the latch unit Lat.

In the first inverter Inv1, the field-effect transistor 213 is turned onor off depending on the first data D1 (DataA). The field-effecttransistor 214 is turned on in response to the control signal WR. Notethat when the field-effect transistor 213 is on, the amount of currentflowing between the source and the drain of the field-effect transistor214 is smaller than the amount of current flowing to the other of thesource and the drain of the field-effect transistor 213.

In the second inverter Inv2, the field-effect transistor 215 is turnedon or off depending on the second data D2 (DataB). The field-effecttransistor 216 is turned on in response to the control signal WR. Notethat when the field-effect transistor 215 is on, the amount of currentflowing between the source and the drain of the field-effect transistor216 is smaller than the amount of current flowing to the other of thesource and the drain of the field-effect transistor 215.

Then, in the hold period (the period T_HLD), the potential of thecontrol signal WR is set at the potential VSL, and the potential of therewrite/read control signal line WRL is set at the potential VSL.

At this time, the field-effect transistors 211 and 212 are turned off,and the first data D1 (DataA) and the second data D2 (DataB) writteninto the latch unit Lat are held by the first inverter Inv1 and thesecond inverter Inv2.

In the first inverter Inv1, the field-effect transistor 213 is turned onor off depending on the first data D1 (DataA). The field-effecttransistor 214 is turned off in response to the control signal WR.

In the second inverter Inv2, the field-effect transistor 215 is turnedon or off depending on the second data D2 (DataB). The field-effecttransistor 216 is turned off in response to the control signal WR.

Then, in a power-off period (also referred to as period T_OFF) withinthe hold period, supply of the power supply voltage to the latch unitLat through the first power supply line PSL1 and the second power supplyline PSL2 stops.

At this time, the first data D1 (DataA) and the second data D2 (DataB)written into the latch unit Lat are held by the first inverter Inv1 andthe second inverter Inv2.

After that, in a power-on period (also referred to as period T_ON),supply of the power supply voltage to the latch unit Lat through thefirst power supply line PSL1 and the second power supply line PSL2starts again.

Then, in the read period (the period T_RD), the potential of the controlsignal WR is set at the potential VSH, and the potential of therewrite/read control signal line WRL is set at the potential VSH.

At this time, the field-effect transistors 211 and 212 are turned on,and the potential of the first data signal line DL1 is set in responseto DataA and the potential of the second data signal line DL2 is set inresponse to DataB. Thus, the first data D1 (DataA) and the second dataD2 (DataB) stored in the latch unit Lat are read.

In the first inverter Inv1, the field-effect transistor 213 is turned onor off depending on the first data D1 (DataA). The field-effecttransistor 214 is turned on in response to the control signal WR. Notethat when the field-effect transistor 213 is on, the amount of currentflowing between the source and the drain of the field-effect transistor214 is smaller than the amount of current flowing to the other of thesource and the drain of the field-effect transistor 213.

In the second inverter Inv2, the field-effect transistor 215 is turnedon or off depending on the second data D2 (DataB). The field-effecttransistor 216 is turned on in response to the control signal WR. Notethat when the field-effect transistor 215 is on, the amount of currentflowing between the source and the drain of the field-effect transistor216 is smaller than the amount of current flowing to the other of thesource and the drain of the field-effect transistor 215.

Accordingly, even if the value of the high-level data, which is one ofthe first data D1 and the second data D2 in the latch unit Lat, ischanged during the hold period, for example, current flows between thesource and the drain of the field-effect transistor 214 or thefield-effect transistor 216 in the read period, whereby the value of thehigh-level data, which is one of the first data D1 and the second dataD2, can be corrected.

The above is the description of the example of the memory circuit inthis embodiment.

As has been described with FIGS. 4A and 4B, FIGS. 5A and 5B, FIGS. 6Aand 6B, FIGS. 7A and 7B, and FIG. 8, in the memory circuit according toone example of this embodiment, the inverter composed of onlytransistors having the same conductivity type in the latch unit Lat isformed using a field-effect transistor, a gate potential of which iscontrolled in accordance with a control signal. Thus, the field-effecttransistor can be off during a non-operating period such as the holdperiod. Consequently, generation of leakage current of the inverter canbe suppressed, so that power consumption of the memory circuit can bereduced. Further, a control signal used for controlling the first switchunit and the second switch unit also controls the inverter, whereby thenumber of signals used in the memory circuit can be decreased.

In one example of this embodiment, the memory circuit is a nonvolatilememory circuit, in which case data can be held even if supply of thepower supply voltage stops, for example. Accordingly, it is possible tostop supply of the power supply voltage to the memory circuit in thehold period, for example; thus, power consumption can be reduced whilesupply of the power supply voltage stops.

Embodiment 3

In this embodiment, an example of the structure of the memory circuitdescribed in Embodiment 2 will be described with reference to FIGS. 9Ato 9C. FIGS. 9A to 9C are schematic cross-sectional views.

As illustrated in FIGS. 9A to 9C, a memory circuit according to oneexample of this embodiment has a structure in which field-effecttransistors 701 and 702 included in the first inverter Inv1 or thesecond inverter Inv2 are stacked over a field-effect transistor 700provided in the switch unit SwA or the switch unit SwB. The field-effecttransistors 700 to 702 have the same conductivity type. The field-effecttransistor 701 has a pair of gates. Here, one of the pair of gates iscalled a first gate and the other thereof is called a second gate. Notethat the structures of the field-effect transistors 700 to 702 are notlimited to those illustrated in FIGS. 9A to 9C.

The memory circuit illustrated in FIGS. 9A to 9C includes an insulatinglayer 711, a semiconductor layer 713, an insulating layer 716, aconductive layer 717, an insulating layer 718, insulating layers 719 aand 719 b, conductive layers 720 a and 720 b, an insulating layer 721,an insulating layer 722, a conductive layer 751, an insulating layer752, an insulating layer 811, a semiconductor layer 813, insulatinglayers 816 a and 816 b, conductive layers 817 a and 817 b, insulatinglayers 818 a and 818 b, insulating layers 819 a to 819 d, conductivelayers 820 a to 820 c, an insulating layer 821, and conductive layers851 a to 851 d.

The insulating layer 711 is provided over a substrate 710. Theinsulating layer 711 functions as a base layer.

The semiconductor layer 713 is provided over the insulating layer 711.The semiconductor layer 713 includes low-resistance regions 714 a and714 b and a channel formation region 715 placed between thelow-resistance regions 714 a and 714 b. The semiconductor layer 713functions as a channel formation layer of the field-effect transistor700.

The insulating layer 716 is provided over the semiconductor layer 713.The insulating layer 716 functions as a gate insulating layer of thefield-effect transistor 700.

The conductive layer 717 overlaps the channel formation region 715 withthe insulating layer 716 placed therebetween. The conductive layer 717functions as a gate of the field-effect transistor 700. Note that theconductive layer 717 serving as the gate may be referred to as a gateelectrode or a gate wiring.

The insulating layer 718 is provided over the conductive layer 717. Theinsulating layer 718 functions as a protective insulating layer of thefield-effect transistor 700.

The insulating layer 719 a is in contact with one of a pair of sidesurfaces of the conductive layer 717. The insulating layer 719 b is incontact with the other of the pair of side surfaces of the conductivelayer 717. The insulating layers 719 a and 719 b function as sidewalls.

The conductive layer 720 a is in contact with the low-resistance region714 a and the insulating layer 719 a. The conductive layer 720 afunctions as one of a source and a drain of the field-effect transistor700.

The conductive layer 720 b is in contact with the low-resistance region714 b and the insulating layer 719 b. The conductive layer 720 bfunctions as the other of the source and the drain of the field-effecttransistor 700.

The insulating layer 721 is provided over the insulating layer 711 so asto fill recessed portions generated due to the field-effect transistor700 and the like. The insulating layer 721 functions as a planarizationlayer.

The insulating layer 722 is provided over the field-effect transistor700 and the insulating layer 721.

The conductive layer 751 is provided over part of the insulating layer722. Moreover, as illustrated in FIG. 9C, the conductive layer 751 is incontact with the conductive layer 717 in a first opening that penetratesthe insulating layer 718 and the insulating layer 722. The conductivelayer 751 functions as the second gate of the field-effect transistor701.

The insulating layer 752 is provided over the insulating layer 722 so asto fill recessed portions generated due to the conductive layer 751 andthe like. The insulating layer 752 functions as a planarization layer.

The insulating layer 811 is provided over the conductive layer 751 andthe insulating layer 752. The insulating layer 811 functions as a baselayer. Note that the insulating layer 811 is thicker than the insulatinglayer 816 a and the insulating layer 816 b in FIGS. 9A and 9B; however,the structure is not limited thereto.

The semiconductor layer 813 is provided over the insulating layer 811.The semiconductor layer 813 includes low-resistance regions 814 a to 814c, a channel formation region 815 a placed between the low-resistanceregions 814 a and 814 c, and a channel formation region 815 b placedbetween the low-resistance regions 814 b and 814 c. The channelformation region 815 a overlaps the conductive layer 751 with theinsulating layer 811 placed therebetween. The semiconductor layer 813functions as a channel formation layer of the field-effect transistors701 and 702.

The insulating layer 816 a is provided over part of the semiconductorlayer 813 to overlap the channel formation region 815 a. The insulatinglayer 816 a functions as a gate insulating layer of the field-effecttransistor 701.

The insulating layer 816 b is provided over part of the semiconductorlayer 813 to overlap the channel formation region 815 b. The insulatinglayer 816 b functions as a gate insulating layer of the field-effecttransistor 702.

The conductive layer 817 a overlaps the channel formation region 815 awith the insulating layer 816 a placed therebetween. The conductivelayer 817 a functions as the first gate of the field-effect transistor701.

The conductive layer 817 b overlaps the channel formation region 815 bwith the insulating layer 816 b placed therebetween. The conductivelayer 817 b functions as a gate of the field-effect transistor 702.

The insulating layer 818 a is provided over the conductive layer 817 a.The insulating layer 818 a functions as a protective insulating layer ofthe field-effect transistor 701.

The insulating layer 818 b is provided over the conductive layer 817 b.The insulating layer 818 b functions as a protective insulating layer ofthe field-effect transistor 702.

The insulating layer 819 a is in contact with one of a pair of sidesurfaces of the conductive layer 817 a. The insulating layer 819 b is incontact with the other of the pair of side surfaces of the conductivelayer 817 a. The insulating layer 819 c is in contact with one of a pairof side surfaces of the conductive layer 817 b. The insulating layer 819d is in contact with the other of the pair of side surfaces of theconductive layer 817 b. The insulating layers 819 a to 819 d function assidewalls.

The conductive layer 820 a is in contact with the low-resistance region814 a and the insulating layer 819 a. The conductive layer 820 afunctions as one of a source and a drain of the field-effect transistor701.

The conductive layer 820 b is in contact with the low-resistance region814 b and the insulating layer 819 c. The conductive layer 820 bfunctions as one of a source and a drain of the field-effect transistor702.

The conductive layer 820 c is in contact with the low-resistance region814 c and the insulating layers 819 b and 819 d. The conductive layer820 c functions as the other of the source and the drain of thefield-effect transistor 701 and the other of the source and the drain ofthe field-effect transistor 702.

The insulating layer 821 is provided over the insulating layer 811 so asto fill recessed portions generated due to the field-effect transistors701 and 702 and the like. The insulating layer 821 functions as aplanarization layer.

The conductive layer 851 a is in contact with the conductive layer 720 ain a second opening that penetrates the insulating layer 721, theinsulating layer 722, the insulating layer 752, the insulating layer811, and the insulating layer 821. The conductive layer 851 a functionsas the data signal line DL.

The conductive layer 851 b is in contact with the conductive layer 720 bin a third opening that penetrates the insulating layer 721, theinsulating layer 722, the insulating layer 752, the insulating layer811, and the insulating layer 821. Moreover, the conductive layer 851 bis in contact with the conductive layer 817 a in a fourth opening thatpenetrates the insulating layer 818 a, and is in contact with theconductive layer 820 c in a fifth opening that penetrates the insulatinglayer 821. The conductive layer 851 b functions as a connection wiring.

The conductive layer 851 c is in contact with the conductive layer 820 ain a sixth opening that penetrates the insulating layer 821. Theconductive layer 851 c functions as the first power supply line PSL1.

The conductive layer 851 d is in contact with the conductive layer 820 bin a seventh opening that penetrates the insulating layer 821. Theconductive layer 851 d functions as the second power supply line PSL2.

Next, the components will be described in detail below.

The substrate 710 can be a glass substrate or a silicon substrate, forexample.

Each of the insulating layers 711 and 811 can be a layer containing amaterial such as silicon oxide, silicon nitride, silicon oxynitride,silicon nitride oxide, aluminum oxide, aluminum nitride, aluminumoxynitride, aluminum nitride oxide, or hafnium oxide, for example.Alternatively, each of the insulating layers 711 and 811 can be a stackof layers that can be used for the insulating layers 711 and 811.

Each of the semiconductor layers 713 and 813 can be a semiconductorlayer of a material with a wider bandgap than silicon (e.g., an oxidesemiconductor layer).

The oxide semiconductor layer is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

Examples of an oxide semiconductor that can be used for thesemiconductor layers 713 and 813 are a metal oxide containing zinc andat least one of indium and gallium, and the metal oxide in which galliumis partly or entirely replaced with another metal element.

As the metal oxide, an In-based metal oxide, a Zn-based metal oxide, anIn—Zn-based metal oxide, or an In—Ga—Zn-based metal oxide can be used,for example. Alternatively, the In—Ga—Zn-based metal oxide in which Ga(gallium) is partly or entirely replaced with another metal element maybe used.

As the aforementioned another metal element, a metal element that iscapable of combining with more oxygen atoms than gallium can be used,for example, and specifically one or more of titanium, zirconium,hafnium, germanium, and tin can be used, for instance. Alternatively, asthe aforementioned another metal element, one or more of lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetiumcan be used. The aforementioned another metal element functions as astabilizer. Note that the amount of the aforementioned another metalelement added is determined so that the metal oxide can function as asemiconductor. When a metal element that is capable of combining withmore oxygen atoms than gallium is used and oxygen is supplied to a metaloxide, oxygen defects in the metal oxide can be reduced.

For example, when tin is used instead of all of Ga (gallium) containedin the In—Ga—Zn-based metal oxide, an In—Sn—Zn-based metal oxide isobtained. When titanium is replaced with part of Ga (gallium) containedin the In—Ga—Zn-based metal oxide, an In—Ti—Ga—Zn-based metal oxide isobtained.

The oxide semiconductor layer may be an oxide semiconductor layercontaining a c-axis aligned crystalline oxide semiconductor (CAAC-OS).

The CAAC-OS is an oxide semiconductor that is not completely singlecrystal nor completely amorphous and has a crystal-amorphous mixed phasestructure including crystal parts in an amorphous phase. In each of thecrystal parts included in the CAAC-OS, the c-axis is aligned in adirection parallel to a normal vector of a surface where the CAAC-OSfilm is formed or a normal vector of a surface of the CAAC-OS film,triangular or hexagonal atomic arrangement which is seen from thedirection perpendicular to the a-b plane is formed, and layerscontaining metal atoms and oxygen atoms are stacked. Note that a normalvector of the layer containing metal atoms and oxygen atoms is thec-axis direction. In this specification, the term “perpendicular”includes a range from 85° to 95°, and the term “parallel” includes arange from −5° to 5°.

In a field-effect transistor including an oxide semiconductor layercontaining the CAAC-OS as a channel formation layer, variations inelectrical characteristics due to irradiation with visible light andultraviolet light are small; therefore, the reliability is high.

In the case where an oxide semiconductor layer is used as thesemiconductor layers 713 and 813, the oxide semiconductor layer can behighly purified in the following manner, for example: dehydration ordehydrogenation is performed so that impurities such as hydrogen, water,a hydroxyl group, and a hydride (also referred to as hydrogen compound)are removed from the oxide semiconductor layer, and oxygen is suppliedto the oxide semiconductor layer. For example, a layer containing oxygenis used as the layer in contact with the oxide semiconductor layer, andheat treatment is performed; thus, the oxide semiconductor layer can behighly purified.

For example, heat treatment is performed at a temperature higher than orequal to 150° C. and lower than the strain point of the substrate,preferably higher than or equal to 350° C. and lower than the strainpoint of the substrate, further preferably ranging from 350° C. to 450°C. Heat treatment may be further performed in a later step. As a heattreatment apparatus for the heat treatment, for example, an electricfurnace or an apparatus for heating an object by heat conduction or heatradiation from a heater such as a resistance heater can be used; forinstance, a rapid thermal annealing (RTA) apparatus such as a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used.

After the heat treatment, a high-purity oxygen gas, a high-purity N₂Ogas, or ultra-dry air (having a dew point of −40° C. or lower,preferably −60° C. or lower) may be introduced in the furnace where theheat treatment has been performed while the heating temperature is beingmaintained or being decreased. In this case, it is preferable that theoxygen gas or the N₂O gas do not contain water, hydrogen, and the like.The purity of the oxygen gas or the N₂O gas which is introduced into theheat treatment apparatus is preferably 6N or higher, more preferably 7Nor higher (i.e., the impurity concentration of the oxygen gas or the N₂Ogas is preferably 1 ppm or lower, more preferably 0.1 ppm or lower). Bythe action of the oxygen gas or the N₂O gas, oxygen is supplied to theoxide semiconductor layer, and defects due to oxygen vacancy in theoxide semiconductor layer can be reduced. Note that the high-purityoxygen gas, high-purity N₂O gas, or ultra-dry air may be introduced atthe time of the above heat treatment.

With the use of the highly purified oxide semiconductor, the carrierdensity of the oxide semiconductor layer in the field-effect transistorcan be lower than 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, furtherpreferably lower than 1×10¹¹/cm³. The off-state current of thefield-effect transistor per micrometer of channel width can be 10 aA(1×10⁻¹⁷ A) or less, preferably 1 aA (1×10⁻¹⁸ A) or less, morepreferably 10 zA (1×10⁻²° A) or less, further preferably 1 zA (1×10⁻²¹A) or less, still more preferably 100 yA (1×10⁻²² A) or less. It ispreferable that the off-state current of the field-effect transistor beas low as possible; the lower limit of the off-state current of thefield-effect transistor is estimated to be approximately 10⁻³° A/μm.

The low-resistance regions 714 a, 714 b, 814 a, 814 b, and 814 c containdopants. As the dopant, one or more of elements of Group 13 in theperiodic table (e.g., boron), elements of Group 15 in the periodic table(e.g., one or more of nitrogen, phosphorus, and arsenic), and rare gaselements (e.g., one or more of helium, argon, and xenon) can be used,for example.

Each of the insulating layers 716, 816 a, and 816 b can be a layercontaining a material such as silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride,aluminum oxynitride, aluminum nitride oxide, or hafnium oxide, forexample. Alternatively, each of the insulating layers 716, 816 a, and816 b can be a stack of layers that can be used for the insulatinglayers 716, 816 a, and 816 b.

Each of the conductive layers 717, 817 a, and 817 b can be a layercontaining a metal material such as molybdenum, titanium, chromium,tantalum, magnesium, silver, tungsten, aluminum, copper, neodymium, orscandium, for example. Alternatively, each of the conductive layers 717,817 a, and 817 b can be a stack of layers that can be used for theconductive layers 717, 817 a, and 817 b.

Each of the insulating layers 718, 818 a, and 818 b can be a layercontaining a material such as silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride,aluminum oxynitride, aluminum nitride oxide, or hafnium oxide, forexample. Alternatively, each of the insulating layers 718, 818 a, and818 b can be a stack of layers that can be used for the insulatinglayers 718, 818 a, and 818 b.

Each of the insulating layers 719 a and 719 b and the insulating layers819 a to 819 d can be a layer containing a material such as siliconoxide, silicon nitride, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitrideoxide, or hafnium oxide, for example. Alternatively, each of theinsulating layers 719 a and 719 b and the insulating layers 819 a to 819d can be a stack of layers that can be used for the insulating layers719 a and 719 b and the insulating layers 819 a to 819 d.

Each of the conductive layers 720 a and 720 b and the conductive layers820 a to 820 c can be a layer containing a metal material such asmolybdenum, titanium, chromium, tantalum, magnesium, silver, tungsten,aluminum, copper, neodymium, scandium, or ruthenium, for example.Alternatively, each of the conductive layers 720 a and 720 b and theconductive layers 820 a to 820 c can be a stack of layers that can beused for the conductive layers 720 a and 720 b and the conductive layers820 a to 820 c.

Each of the insulating layers 721, 722, and 821 can be a layercontaining a material such as silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride,aluminum oxynitride, aluminum nitride oxide, or hafnium oxide, forexample. Alternatively, each of the insulating layers 721, 722, and 821can be a stack of layers that can be used for the insulating layers 721,722, and 821.

The conductive layer 751 can be a layer containing a metal material suchas molybdenum, titanium, chromium, tantalum, magnesium, silver,tungsten, aluminum, copper, neodymium, scandium, or ruthenium, forexample. Alternatively, the conductive layer 751 can be a stack oflayers that can be used for the conductive layer 751.

The insulating layer 752 can be a layer containing a material such assilicon oxide, silicon nitride, silicon oxynitride, silicon nitrideoxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminumnitride oxide, or hafnium oxide, for example. Alternatively, theinsulating layer 752 can be a stack of layers that can be used for theinsulating layer 752.

Each of the conductive layers 851 a to 851 d can be a layer containing ametal material such as molybdenum, titanium, chromium, tantalum,magnesium, silver, tungsten, aluminum, copper, neodymium, scandium, orruthenium, for example. Alternatively, each of the conductive layers 851a to 851 d can be a stack of layers that can be used for the conductivelayers 851 a to 851 d.

The above is the description of the structure example of the memorydevice illustrated in FIGS. 9A to 9C.

As described with reference to FIGS. 9A to 9C, the memory circuitaccording to one example of this embodiment is composed by stacking aplurality of field-effect transistors, resulting in reduction in thecircuit area.

Embodiment 4

In this embodiment, as a memory device including the above-describedmemory circuit, an example of static random access memory (SRAM) will bedescribed with reference to FIG. 10.

A memory device illustrated in FIG. 10 includes a driver circuit 901, adriver circuit 902, and a plurality of memory cells 910 arranged in Xrows and Y columns (each of X and Y is a natural number of 2 or more).

The driver circuit 901 is supplied with a data signal and a columnaddress signal. The driver circuit 901 selects a column address of amemory cell 910 that is a target for data rewriting or reading bycontrolling the potentials of first data signal lines DL1_1 to DL1_Y andsecond data signal lines DL2_1 to DL2_Y. The driver circuit 901 includesa column decoder and a sense amplifier.

The driver circuit 902 is supplied with a row address signal. The drivercircuit 902 selects one of rewrite/read control signal lines WRL_1 toWRL_X in response to an inputted row address signal, and selects a rowaddress of the memory cell 910 that is the target for data rewriting orreading by controlling the potential of the selected rewrite/readcontrol signal line WRL. The driver circuit 902 includes a row decoder.

As the memory cell 910, any of the memory circuits described inEmbodiments 1 and 2 can be used, for example. In this case, the firstdata signal line DL1 electrically connected to the memory cell 910 inthe M-th row (M is a natural number of X or less) and the N-th column (Nis a natural number of Y or less) is the first data signal line DL1_N.Moreover, the second data signal line DL2 electrically connected to thememory cell 910 in the M-th row and the N-th column is the second datasignal line DL2_N. The rewrite/read control signal line WRL electricallyconnected to the memory cell 910 in the M-th row and the N-th column isthe rewrite/read control signal line WRL_M. The memory cell 910 isselected by the driver circuit 901 and the driver circuit 902, and theselected memory cell 910 is subjected to data rewriting or reading.

The above is the description of the example of the memory device in thisembodiment.

According to one example of this embodiment, a low-power memory devicecan be provided by using the memory circuit in any of the aboveembodiments as a memory cell of SRAM as has been described with FIG. 10.

In addition, according to one example of this embodiment, a nonvolatilememory device can be provided by using the nonvolatile memory circuit inany of the above embodiments as a memory cell of SRAM.

Embodiment 5

In this embodiment, an example of an arithmetic processing unit, such asa CPU, including the memory device shown in Embodiment 4 as a memorywill be described.

An example of the arithmetic processing unit in this embodiment isdescribed with reference to FIG. 11.

The arithmetic processing unit illustrated in FIG. 11 includes a businterface 951, a control circuit 952, a cache memory 953, first to Z-thregisters 954 (Z is a natural number of 3 or more), an instructiondecoder 955, and an arithmetic logic unit 956.

The bus interface 951 has a function of exchanging signals with theoutside of the arithmetic processing unit and a function of exchangingsignals with the circuits in the arithmetic processing unit, forexample.

The control circuit 952 has a function of controlling operations of thecircuits in the arithmetic processing unit.

The control circuit 952 can include an integrated circuit, for example.

The cache memory 953 is controlled by the control circuit 952 and has afunction of temporarily retaining data used while the arithmeticprocessing unit operates. Note that the arithmetic processing unit mayinclude a plurality of cache memories 953 as a primary cache and asecondary cache, for example.

As the cache memory 953, the memory device shown in Embodiment 4 can beused, for example.

The first to Z-th registers 954 are controlled by the control circuit952 and have a function of storing data used for arithmetic processing.For example, one register 954 may be used as a register for thearithmetic logic unit 956 and another register 954 may be used as aregister for the instruction decoder 955.

The instruction decoder 955 has a function of translating a readinstruction signal. The translated instruction signal is input to thecontrol circuit 952, and the control circuit 952 outputs a controlsignal corresponding to the instruction signal to the arithmetic logicunit 956.

The arithmetic logic unit 956 is controlled by the control circuit 952and has a function of performing logical operation in response to theinputted instruction signal.

According to one example of this embodiment, a low-power arithmeticprocessing unit can be provided by using the memory device in Embodiment4 as a cache memory as has been described with FIG. 11.

In the arithmetic processing unit according to one example of thisembodiment, by using the nonvolatile memory device in Embodiment 4 asthe cache memory, part of internal data existing just before supply ofthe power supply voltage stops can be retained in the cache memory evenwhen supply of the power supply voltage stops, and when supply of thepower supply voltage restarts, the state of the arithmetic processingunit can be returned to the one immediately before stop of supply of thepower supply voltage. Accordingly, even if supply of the power supplyvoltage stops temporarily, the time after supply of the power supplyvoltage restarts until the normal operation starts can be shortened.

Embodiment 6

In this embodiment, examples of electronic devices including thearithmetic processing unit in Embodiment 5 in a calculation unit will bedescribed with reference to FIGS. 12A to 12D and FIG. 13.

FIGS. 12A to 12D each illustrate the appearance of an electronic devicein this embodiment.

The electronic device illustrated in FIG. 12A is an example of apersonal digital assistant.

The electronic device illustrated in FIG. 12A includes a housing 1011, apanel 1012 incorporated in the housing 1011, a button 1013, and aspeaker 1014.

The housing 1011 may be provided with a connection terminal forconnecting the electronic device in FIG. 12A to an external deviceand/or a button used to operate the electronic device in FIG. 12A.

The panel 1012 functions as a display panel and a touch panel.

The button 1013 is provided on the housing 1011. For example, when thebutton 1013 serving as a power button is provided, the electronic devicecan be turned on or off by pressing the button 1013.

The speaker 1014 is provided on the housing 1011. The speaker 1014 has afunction of outputting sound.

The housing 1011 may be provided with a microphone, in which case theelectronic device in FIG. 12A can function as a telephone, for example.

The electronic device illustrated in FIG. 12A has a function of at leastone of a telephone, an e-book reader, a personal computer, and a gamemachine, for example.

The electronic device illustrated in FIG. 12B is an example of afoldable personal digital assistant.

The electronic device illustrated in FIG. 12B includes a housing 1021 a,a housing 1021 b, a panel 1022 a incorporated in the housing 1021 a, apanel 1022 b incorporated in the housing 1021 b, a hinge 1023, a button1024, a connection terminal 1025, a storage medium insertion portion1026, and a speaker 1027.

The housing 1021 a and the housing 1021 b are connected with the hinge1023.

Each of the panels 1022 a and 1022 b functions as a display panel and atouch panel.

In the electronic device illustrated in FIG. 12B, the housing 1021 a canbe made to overlap the housing 1021 b, for example, by moving thehousing 1021 a or the housing 1021 b with the use of the hinge 1023, sothat the electronic device can be folded.

The button 1024 is provided on the housing 1021 b. Note that the button1024 may be provided on the housing 1021 a. When the button 1024 havinga function of a power button is provided, whether to supply power to acircuit in the electronic device can be controlled by pressing thebutton 1024.

The connection terminal 1025 is provided on the housing 1021 a. Notethat the connection terminal 1025 may be provided on the housing 1021 b.Alternatively, a plurality of connection terminals 1025 may be providedon one or both of the housings 1021 a and 1021 b. The connectionterminal 1025 is a terminal for connecting the electronic device in FIG.12B to another device.

The storage medium insertion portion 1026 is provided on the housing1021 a. Note that the storage medium insertion portion 1026 may beprovided on the housing 1021 b. Alternatively, a plurality of storagemedium insertion portions 1026 may be provided on one or both of thehousings 1021 a and 1021 b. For example, when a card storage medium isinserted into the storage medium insertion portion 1026, data can beread from the card storage medium and sent to the electronic device, ordata stored in the electronic device can be written into the cardstorage medium.

The speaker 1027 is provided on the housing 1021 b. The speaker 1027 hasa function of outputting sound. Note that the speaker 1027 may beprovided on the housing 1021 a instead of on the housing 1021 b.

The housing 1021 a or the housing 1021 b may be provided with amicrophone, in which case the electronic device in FIG. 12B can functionas a telephone, for example.

The electronic device illustrated in FIG. 12B has a function of at leastone of a telephone, an e-book reader, a personal computer, and a gamemachine, for example.

The electronic device illustrated in FIG. 12C is an example of astationary information terminal. The stationary information terminalillustrated in FIG. 12C includes a housing 1031, a panel 1032incorporated in the housing 1031, a button 1033, and a speaker 1034.

The panel 1032 functions as a display panel and a touch panel.

Note that the panel 1032 may be provided on a top board 1035 of thehousing 1031.

Further, the housing 1031 may be provided with at least one of a ticketslot for issuing a ticket or the like, a coin slot, and a bill slot.

The button 1033 is provided on the housing 1031. When the button 1033having a function of a power button is provided, whether to supply powerto a circuit in the electronic device can be controlled by pressing thebutton 1033.

The speaker 1034 is provided on the housing 1031. The speaker 1034 has afunction of outputting sound.

The electronic device illustrated in FIG. 12C serves as an automatedteller machine, an information communication terminal (also referred toas multimedia station) for ordering a ticket or the like, or a gamemachine, for example.

FIG. 12D illustrates an example of a stationary information terminal.The electronic device illustrated in FIG. 12D includes a housing 1041, apanel 1042 incorporated in the housing 1041, a support 1043 forsupporting the housing 1041, a button 1044, a connection terminal 1045,and a speaker 1046.

Note that the housing 1041 may be provided with a connection terminalfor connecting the electronic device in FIG. 12D to an external deviceand/or a button for operating the electronic device in FIG. 12D.

The panel 1042 functions as a display panel. The panel 1042 may alsofunction as a touch panel.

The button 1044 is provided on the housing 1041. When the button 1044having a function of a power button is provided, whether to supply powerto a circuit in the electronic device can be controlled by pressing thebutton 1044.

The connection terminal 1045 is provided on the housing 1041. Theconnection terminal 1045 is a terminal for connecting the electronicdevice in FIG. 12D to another device. For example, when the electronicdevice in FIG. 12D and a personal computer are connected with theconnection terminal 1045, the panel 1042 can display an imagecorresponding to a data signal input from the personal computer. Forexample, when the panel 1042 of the electronic device in FIG. 12D islarger than a panel of another electronic device connected thereto, adisplayed image of the other electronic device can be enlarged, so thata plurality of viewers can easily see the image at the same time.

The speaker 1046 is provided on the housing 1041. The speaker 1046 has afunction of outputting sound.

The electronic device illustrated in FIG. 12D functions as an outputmonitor, a personal computer, or a television set, for example.

FIG. 13 illustrates an example of circuit blocks in the electronicdevices illustrated in FIGS. 12A to 12D.

The electronic device illustrated in FIG. 13 includes a communicationunit 1101, a power source unit 1102, a calculation unit 1103, an audiounit 1104, and a panel unit 1105.

The communication unit 1101 has a function of transmitting and receivingdata. For example, in order to perform wireless communication, thecommunication unit 1101 is provided with an antenna, a demodulatorcircuit, a modulator circuit, and the like. In this case, thecommunication unit 1101 exchanges data with the outside by transmittingand receiving radio waves with the antenna. Note that a plurality ofantennas may be provided in the communication unit 1101. Thecommunication unit 1101 may transmit and receive data through wiredcommunication.

The power source unit 1102 has a function of supplying power foroperating the electronic device. For example, power is supplied from thepower source unit 1102 to the communication unit 1101, the calculationunit 1103, the audio unit 1104, and the panel unit 1105. Note that apower storage device may be provided in the power source unit 1102, inwhich case the power storage device is provided inside the housing ofthe electronic device. A power source circuit that generates the powersupply voltage for operating the electronic device may be provided inthe power source unit 1102. When the power storage device is provided inthe power source unit 1102, the power supply voltage is generated in thepower source circuit by using power supplied from the power storagedevice. In the case where the power storage device is provided, theelectronic device can be driven by using the power storage device as apower source even if there is no supply of power from the commercialpower supply because of a power failure or the like, for example.

The calculation unit 1103 has a function of performing arithmeticprocessing in response to instruction signals based on data of datasignals input from the communication unit 1101, the audio unit 1104, andthe panel unit 1105, for example. The calculation unit 1103 is providedinside the housing of the electronic device, for example.

The arithmetic processing unit described in Embodiment 5 is provided inthe calculation unit 1103.

The audio unit 1104 has a function of controlling input and output ofsound that is audio data. For example, the audio unit 1104 controlsoutput of sound from a speaker. When the electronic device includes amicrophone, the audio unit 1104 has a function of controlling input ofsound from the microphone.

The panel unit 1105 has a function of controlling the operation of thepanel in the electronic device. For example, the panel unit 1105 may beprovided with a driver circuit for controlling drive of the panel sothat the operation of the panel is controlled.

Note that a control circuit may be provided in at least one of thecommunication unit 1101, the power source unit 1102, the calculationunit 1103, the audio unit 1104, and the panel unit 1105 to control theoperation of each circuit block. Further, a control circuit may beprovided in the calculation unit 1103 to control the operation of one ormore of the communication unit 1101, the power source unit 1102, theaudio unit 1104, and the panel unit 1105.

A memory circuit may be provided in one or more of the communicationunit 1101, the power source unit 1102, the audio unit 1104, and thepanel unit 1105 so as to store data necessary for the operation of eachunit. Thus, the operation speed can be increased.

This application is based on Japanese Patent Applications serial No.2011-256890 filed with Japan Patent Office on Nov. 25, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A memory circuit comprising: a latch unit; afirst switch unit between a first signal line and the latch unit; asecond switch unit between a second signal line and the latch unit; anda third switch unit between the latch unit and a power supply line,wherein the latch unit comprises a first inverter and a second inverter,an input terminal of the first inverter is electrically connected to anoutput terminal of the second inverter, and an input terminal of thesecond inverter is electrically connected to an output terminal of thefirst inverter, wherein the first inverter comprises a field-effecttransistor, and wherein each of a gate of the field-effect transistor,the first switch unit, the second switch unit, and the third switch unitis supplied with a control signal.
 2. The memory circuit according toclaim 1, wherein all of field-effect transistors included in the latchunit, the first switch unit, the second switch unit, the third switchunit have an off-state current of 1 zA or less per channel width of 1μm.
 3. The memory circuit according to claim 1, wherein each offield-effect transistors included in the latch unit, the first switchunit, the second switch unit, the third switch unit includes an oxidesemiconductor layer.
 4. A memory device comprising: memory cells of Xrows and Y columns, where each of X and Y is a natural number of 2 ormore, each of the memory cells having the memory circuit according toclaim 1; a first driver circuit configured to select a column address ofa memory cell targeted for data rewriting or reading; and a seconddriver circuit configured to select a row address of the memory celltargeted for data rewriting or reading.
 5. The memory circuit accordingto claim 1, wherein each of the first inverter and the second invertercomprises: a first field-effect transistor having a first terminal, asecond terminal, and a first gate serving as the input terminal, and asecond field-effect transistor having a same conductivity type as thefirst field-effect transistor, and having a third terminal, a fourthterminal, a second gate electrically connected to the third terminal andthe output terminal, and a third gate supplied with the control signal.6. The memory circuit according to claim 1, wherein each of the firstinverter and the second inverter comprises: a first field-effecttransistor having a first terminal serving as the output terminal, asecond terminal, and a first gate serving as the input terminal, and asecond field-effect transistor having a same conductivity type as thefirst field-effect transistor, and having a third terminal, a fourthterminal electrically connected to the first terminal, and a second gatesupplied with the control signal.
 7. A memory circuit comprising: alatch unit; a first switch unit between a first signal line and thelatch unit; a second switch unit between a second signal line and thelatch unit; and a third switch unit, wherein the latch unit comprises afirst inverter and a second inverter, an input terminal of the firstinverter is electrically connected to an output terminal of the secondinverter, and an input terminal of the second inverter is electricallyconnected to an output terminal of the first inverter, wherein the firstinverter comprises a field-effect transistor, wherein each of a gate ofthe field-effect transistor, the first switch unit, the second switchunit, and the third switch unit is supplied with a control signal, andwherein the third switch unit is configured to control whether to supplypower supply voltage to the latch unit in response to the controlsignal.
 8. The memory circuit according to claim 7, wherein all offield-effect transistors included in the latch unit, the first switchunit, the second switch unit, the third switch unit have an off-statecurrent of 1 zA or less per channel width of 1 μm.
 9. The memory circuitaccording to claim 7, wherein each of field-effect transistors includedin the latch unit, the first switch unit, the second switch unit, thethird switch unit includes an oxide semiconductor layer.
 10. A memorydevice comprising: memory cells of X rows and Y columns, where each of Xand Y is a natural number of 2 or more, each of the memory cells havingthe memory circuit according to claim 7; a first driver circuitconfigured to select a column address of a memory cell targeted for datarewriting or reading; and a second driver circuit configured to select arow address of the memory cell targeted for data rewriting or reading.11. The memory circuit according to claim 7, wherein each of the firstinverter and the second inverter comprises: a first field-effecttransistor having a first terminal, a second terminal, and a first gateserving as the input terminal, and a second field-effect transistorhaving a same conductivity type as the first field-effect transistor,and having a third terminal, a fourth terminal, a second gateelectrically connected to the third terminal and the output terminal,and a third gate supplied with the control signal.
 12. The memorycircuit according to claim 7, wherein each of the first inverter and thesecond inverter comprises: a first field-effect transistor having afirst terminal serving as the output terminal, a second terminal, and afirst gate serving as the input terminal, and a second field-effecttransistor having a same conductivity type as the first field-effecttransistor, and having a third terminal, a fourth terminal electricallyconnected to the first terminal, and a second gate supplied with thecontrol signal.